In recent years, requirement for a reduction in fuel consumption of cars is intensifying in relation to a global movement toward regulation of carbon dioxide emission, which is led from social requirement for energy saving and an increasing interest in environmental issues. To meet such requirement, improvement of tire property, more particularly, a reduction in rolling resistance of a tire has been demanded. A method involving optimizing a tire structure has been studied as a method of reducing the rolling resistance of a tire, but a method which uses a material having low heat generation property as a rubber composition is employed as a most typical method.
To obtain such a rubber composition which generates less heat, a large number of techniques for enhancing the dispersibility of a filler used for a rubber composition has been developed. Among such techniques, particularly a method in which a polymerizable active end of a diene base polymer obtained by anion polymerization using an organic lithium compound is modified with a functional group having interaction with a filler is becoming the most popular method.
As such methods, there are disclosed a method in which carbon black is used for a filler to modify a polymerizable active end with a tin compound (see, for example, Patent Document 1) and a method in which carbon black is used in the same manner to introduce an amino group into a polymerizable active end (see, for example, Patent Document 2). Those methods are capable of increasing the dispersibility of carbon black. However, a further increase in dispersibility of carbon black is demanded.
On the other hand, a modified polymer which is obtained by introducing alkoxysilane having a dialkylamino group into an active end of a polymer obtained by anion polymerization using alkyllithium or lithium amide as a polymerization initiator, is disclosed (see, for example, Patent Documents 3 and 4). However, use of the above modified polymer, of which the amino group is substituted with a dialkyl group having less effect to carbon black, fails to provide sufficient effects particularly for blends that contain a large amount of carbon black, as compared with the case of using a modified polymer obtained by using a tin-based modifier.
Further, a modified polymer using aziridine (ethyleneimine or propyleneimine) as a modifier is known. In this case, aziridine is ring-opened by the modification reaction, so the modified polymer contains no aziridine structure.    Patent Document 1: JP 05-87530 B    Patent Document 2: JP 62-207342 A    Patent Document 3: JP 06-53763 B    Patent Document 4: JP 06-57767 B